Physiological data can be measured from a user by using portable biometric monitors, which may be attached to the user, for example to the wrist, forearm, or arm of the user. The physiological data may include for example heart rate. Traditional monitors usually contain a separate sensor, which is attached for example to the user's chest with a strap, and which communicates wirelessly with the wrist device. The use of separate sensors complicates the use of a portable biometric monitor, and therefore there is a need to develop solutions embedded to the wrist-attached or other extremity attached device.
One way for measuring the heart rate is using optical measurement. The optical heart rate measurement is based on the fact that light is emitted by a light source towards body tissue and at least one detector is configured to detect the intensity of reflected light after propagation through the body tissue.
In such measurement a photoplethysmogram (PPG) is obtained. It is an optically obtained plethysmogram, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption. With each cardiac cycle the heart pumps blood to the periphery. Even though this pressure pulse is somewhat damped by the time it reaches the skin, it is enough to distend the arteries and arterioles in the subcutaneous tissue. If the pulse oximeter is attached without compressing the skin, a pressure pulse can also be seen from the venous plexus, as a small secondary peak.
The change in volume caused by the pressure pulse may be detected by illuminating the skin with the light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode.
Each cardiac cycle appears as a downward peak in the photodiode. Because blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, and other circulatory conditions. Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the pulse oximeter is attached.
There are several challenges when measuring pulse optically. The optical measurement is based on light absorption changes caused by blood flow in a lighted area. If the shape of the lighted area changes during the measurement, for example, due to movement of the pulse measuring device, the measurement is disturbed. Thus, for example, movements of a hand and of the user cause errors to the measurement in many ways.
In order to avoid problems in the measurement of biometric monitors, especially in optical measurements, the device needs to be as stable as possible in relation to skin and needs to minimize mechanical changes in tissue area during movement. This is especially important during activities, such as sports-related activities and workouts, and when the biometric monitoring device is used as an athletic performance or fitness monitor.
The wrists of different users may vary in size ranging in perimeter for example from 12 cm to over 20 cm. This makes it very challenging to optimize the contact of the portable biometric monitor for use with all or most of the users, as the devices are usually produced in one size only.
There are many ways to address the above problems. One solution is to tighten a strap of the measuring device. The problem, however, is that a user may tighten the strap too much, which in turn is uncomfortable and prevents blood flow in tissue. In turn, too loose tightening of the strap allows the portable measuring device to move too much in relation, for example, to a wrist and body tissue. Further, too complicated tightening and setting procedure makes the device less convenient to use.